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If you can't pull it out the top try tapping/driving the bottom of each side of the horseshoe clip up from the bottom. You can use a small thin screwdriver. I assume you turned the water off first.
How about a picture of this problem clip? I've replaced hundreds of these cartridges and never had this problem. You can post a pic here by clicking on ADD FILES next to the SEND button
PB Blaster or even white vinegar will work too. You can get PB Blaster at most Lowes or Home Depots